Methyl methacrylic resins containing methyl methacrylate as a main component have found applications in variety of fields such as lighting apparatuses, parts for automobiles, billboards and construction materials, since the resin is advantageous in regards to transparency, luster on the surface and the like.
The methyl methacrylic resins are not always sufficient in view of scratch-resistance and hardness of the surface. To improve the scratch-resistance, a variety of methods such as a method of applying or laminating a hard-coat layer to the resin surface, has been conducted to prevent scratching. However, these methods consist of many steps before obtaining the products and, are therefore unnecessarily costly for industrial production.
On the other hand, methods wherein methyl methacrylic resin compositions are obtained by cross-linking the resin itself have been proposed for improving the scratch-resistance of the molded article thereof.
For example, the Japanese Patent Laid-Open No. 6-62380-B (corresponding to the U.S. Pat. No. 4,698,373) discloses prosthetic teeth (i) made of a hardened polymer made of composition containing a blend of (a) from 0% to about 50% by weight of an uncross-linked polymer solution, (b) from about 2% to about 30% by weight of a polymerizable monomer, (c) from about 10% to about 70% of a cross-linked polymer in the form of discrete particles having average diameters from about 0.001 micron to about 500 microns and being swollen in said solution and (d) from 20% to about 70% of a cross-linking agent for said monomer, provided that when the component (d) is of from 20% to 27%, the component (b) is of 20% or less and that the above percentages are based on the total weight of components (a), (b), (c) and (d) in said composition; (ii) possessing a superior chemical resistance against methyl methacrylate; and (iii) having a grind-resistance value from 450 to 550 g/sec. It discloses a use of the above-mentioned methyl methacrylic resin composition which somewhat improves the scratch-resistance thereof. The resin composition is used for prosthetic teeth which are made of a stable one part dental prosthesis composition employing an IPN technology.
Additionally, various methods for obtaining alkyl methacrylic resin compositions such as methyl methacrylic resin compositions have been proposed.
For example, the Japanese Patent Application Laid-Open No. 62-2201-A discloses a methyl methacrylic resin composition obtained by cross-linking the resin itself such that the resin composition is obtained by polymerizing a mixture which comprises (a) resin raw materials selected from an alkyl methacrylate monomer, an .alpha.,.beta.-ethylenic unsaturated monomer mixture containing mainly alkyl methacrylates, and a syrup containing a polymer thereof, and (b) 12 to 40 parts by weight of a cross-linking agent based on 100 parts by weight of the resin raw materials. It discloses the resin composition is used for production of a lens for lamp having a thermal deformation temperature of 130.degree. C. or more.
The Japanese Patent Application Laid-Open No. 62-79206-A discloses another methyl methacrylic resin composition obtained by cross-linking the resin itself, and discloses that methacrylic resin moldings are produced by molding partially-crosslinked gel-like polymer prepared by polymerizing partially mixtures of 100 parts by weight of (a) resin raw materials selected from an alkyl methacrylate monomer, an .alpha.,.beta.-ethylenic unsaturated monomer mixture containing mainly alkyl methacrylates, and a syrup containing a polymer thereof, and (b) 2 to 250 parts by weight of a cross-linking agent based on 100 parts by weight of the resin raw materials, wherein the total amounts of the polymers contained in crosslinked polymers are not more than 80 weight % and they are 4 to 65 weight % more than the amounts of the polymers contained in the said mixture of (a) and (b), by a injection molding method, a compression molding method or a transition molding method.
The Japanese Patent Application Laid-Open No. 7-70236-A also discloses another methyl methacrylic resin composition obtained by cross-linking the resin itself, and discloses thermosetting resin compositions containing (i) partially crosslinked gel-like polymers prepared by polymerising (a) monomers containing mainly alkyl methacrylates or syrups thereof and (b) at least two (meth)acryloyl groups-containing compounds and (ii) peroxides containing mainly peroxy-ketals having exothermic peak temperature of at least 110.degree. C. and below 130.degree. C. in amount of 0.05-0.2% in terms of active oxygen.
However, in the Japanese Patent Application Laid-Open No. 6-62380-A, the objects of the invention are that the methyl methacrylic resin composition is used for prosthetic dental appliances such as prosthetic teeth and, therefore, it is suggested that the component is used with pigments and fillers to improve the appearance, the density and the physical properties of the desirable prosthetic teeth, and is not needed that the article made of the resin composition has high transparency.
In all of the Japanese Patent Application Laid-Open Nos. 62-2201-A, 62-79206-A and 7-70236-A, the method of partially polymerizing the methyl methacrylic resin components in the composition are carried out. In such a method, the reaction conditions such as temperature and time have to be elaborately controlled since the content of the polymer is likely to vary widely and this amount of scattering in the content may cause troubles when the composition are molded. Additionally, even once the polymerization reaction is terminated, the reaction might be started again by the remaining polymerization initiator resulting in an unintended composition when the resin composition is transported or is stocked as it is. Special apparatuses or facilities such as an apparatus for keeping cold are needed since a preservative property is not enough and, therefore, the method is disadvantageous, industrially.